Apparatus and method for compensating for sensitivity fluctuations of a magnetic field sensor circuit

This application claims priority to German Patent Application No. 102022121887.2 filed on Aug. 30, 2022, the content of which is incorporated by reference herein in its entirety.

The present disclosure relates to magnetic sensors in general and in particular to compensation for sensitivity drift of magnetoresistive sensors.

Magnetoresistive effects are all effects that describe the change in an electrical resistance of a material by applying an external magnetic field. These include, in particular, the anisotropic magnetoresistive effect (AMR effect), the giant magnetoresistance (GMR effect), the colossal magnetoresistive effect (CMR effect), the magnetic tunnel resistance (TMR effect) and the planar Hall effect. The various magnetoresistive resistances are commonly also referred to as xMR resistances below.

Magnetoresistive sensors (e.g., xMR) sensors are usually realized in the form of half-bridge or full-bridge circuits, whereby the latter can provide a differential sensor signal. Linear xMR sensor bridges may suffer from sensitivity drift caused by mechanical stress (lifetime stability problem), temperature changes, and/or incomplete temperature compensation. Technological variation is also sometimes high and should be trimmed. It is known to trim the sensor sensitivity over the temperature, but this does not solve the drift problem.

Previous stress compensation methods are based on a known correlation of a stress sensor to the sensitivity of a magnetic field sensor on the chip. This correlation is used to compensate for the drift (due to mechanical stress changes). But now there are a wide variety of stress components:

Not all components can be taken into account, which would be too expensive, and in addition correlation factors are also influenced by technological and manufacturing-related fluctuations. In addition, temperature coefficients and correlations must be determined very precisely in laboratory experiments using a sample, since no stress fluctuations (e.g., due to changes in moisture in the plastic package of the IC, or soldering the IC onto a printed circuit board) can be measured individually for each IC during production. Therefore, there is reliance on statistical mean values of the correlation coefficients and their temperature profile, without being able to take into account their manufacturing-related tolerances.

Therefore, there is a need for magnetic sensor apparatuses that can compensate for the sensitivity drift.

This need is taken into account using apparatuses and methods according to the independent patent claims. The dependent claims relate to advantageous developments.

A first aspect of the present disclosure proposes a magnetic sensor apparatus. The magnetic sensor apparatus includes a magnetic field generating circuit which is configured to generate a magnetic field. The magnetic field can be predefined (e.g., known). The magnetic sensor apparatus includes a magnetic field sensor circuit which is configured to output a sensor signal in response to the magnetic field, which sensor signal has a signal amplitude dependent on a sensitivity (measurement sensitivity) of the magnetic field sensor circuit. The magnetic sensor apparatus includes an amplifier circuit which is configured to amplify the sensor signal and to output an amplified sensor signal with an amplified signal amplitude. In addition, the magnetic sensor apparatus includes a control circuit which is configured to use a setting signal to set a supply signal (supply current, supply voltage) of the magnetic field sensor circuit and/or a gain of the amplifier circuit such that the amplified signal amplitude corresponds to or approaches a target amplitude. A sensitivity drift of the magnetic field sensor circuit can be compensated for by adjusting the supply signal and/or gain to the target amplitude.

According to some example implementations, the magnetic field sensor circuit includes at least one Hall sensor.

Alternatively, the magnetic field sensor circuit may include at least one xMR sensor. The xMR sensor may be in the form of an xMR sensor bridge circuit and in particular a TMR sensor bridge circuit.

According to some example implementations, the magnetic field generating circuit includes a current conductor for an electrical excitation current. For example, the current conductor and the magnetic field sensor circuit can be arranged on a common semiconductor chip. The current conductor is also referred to as a so-called Wire on Chip (WoC), in order to generate the magnetic field using the electrical excitation current. The amplifier circuit and the control circuit can also be integrated on the chip.

According to some example implementations, the magnetic field generating circuit is configured to generate the magnetic field using an AC excitation current (AC=alternating current) as an AC magnetic field. Such an AC magnetic field for trimming the magnetic field sensor circuit can be superimposed, for example, on a DC magnetic field (DC=direct current) to be measured.

According to some example implementations, the magnetic field generating circuit is configured to generate the (AC) magnetic field according to a spreading code. For example, the electrical excitation current for the magnetic field may be modulated according to the spreading code. This is similar to a band spreading method such as CDMA (Code Division Multiple Access) or DSSS (Direct Sequence Spread Spectrum), where a message to be transmitted is spread with the spreading code which consists of a sequence of chips or pulses.

According to some example implementations, the magnetic field generating circuit is configured to generate the predefined magnetic field according to a pseudo-random pilot signal (e.g., pseudo-random pilot signal=spreading code). For example, the electrical excitation current can be modulated according to the pseudo-random pilot signal. The pseudo-random pilot signal can be a digital n-bit signal, with n≥1. For example, it may be a binary pseudo-random pilot signal.

According to some example implementations, the magnetic sensor apparatus further includes a removal circuit which is configured to remove the spreading code or the pseudo-random pilot signal at least partially from the amplified sensor signal. For this purpose, the removal circuit may be configured, for example, to demodulate the amplified sensor signal accordingly. Additionally, or alternatively, the removal circuit may have an n-bit digital-to-analog converter (DAC) for the pseudo-random pilot signal and may be configured to subtract an output signal of the n-bit DAC from the amplified sensor signal, where n corresponds to a number of bits of the spreading code or pseudo-random pilot signal. It is understood that the magnetic field generating circuit and the removal circuit should be synchronized with regard to the spreading code or the pseudo-random pilot signal. Alternative implementations of the removal circuit based on low-pass filters or bandpass filters are also possible.

According to some example implementations, the magnetic sensor apparatus further includes a bandgap reference circuit which is configured to generate a constant reference voltage with the target amplitude and/or an electrical excitation current for the magnetic field generating circuit with a constant current intensity. A bandgap reference (bandgap voltage reference) is a reference voltage source whose output voltage in the temperature-compensated state corresponds to the bandgap voltage of a semiconductor. The voltage generated thus varies depending on the semiconductor material: silicon, silicon carbide or gallium arsenide. A special feature of a bandgap reference is high precision with low outlay in terms of circuitry. In addition, bandgap references are temperature-stable.

According to some example implementations, the bandgap reference circuit has a resistor (virtually) independent of mechanical stress in order to generate a current (virtually) independent of mechanical stress. This can be, for example, a metal resistor or a silicided polysilicon resistor or combinations of resistors with different stress dependence, wherein the combination has a stress dependence <1%. Integrated circuits (ICs) often require integrated resistors for proper operation of the circuit. Such resistors are usually composed of doped polycrystalline silicon. In order to reduce the resistances and the dependence on mechanical stress in the polycrystalline resistors, a metal silicide layer can be formed on the top side of the doped polycrystalline silicon or the formation of such a layer on the top side of the doped polycrystalline silicon can be prevented. This option of the metal silicide allows two different types of resistors made of polycrystalline silicon. The first type with the metal silicide layer above the doped polycrystalline silicon is referred to as a “silicided polycrystalline silicon resistor” (silicided poly-resistor), and the electrical conduction of this resistor is effected via the metal silicide layer. The second type without the metal silicide layer above the doped polycrystalline silicon is referred to as a “non-silicided polycrystalline silicon resistor” (non-silicided poly-resistor). The electrical conduction of the second type is effected through the polycrystalline silicon, which depends on the doping (p-doped or n-doped) of the polycrystalline silicon.

According to some example implementations, the control circuit has a differential amplifier which is configured to output the setting signal for the supply signal and/or the amplifier circuit, based on a difference between the amplified signal amplitude and the target amplitude. The differential amplifier can be, for example, an operational amplifier or transconductance amplifier (Operational Transconductance Amplifier, OTA) which converts a differential voltage at its two inputs into a proportional output current (setting signal). An analog-to-digital converter is also possible for signal processing, the reference of which converter is controlled. This is also equivalent to gain or sensitivity control.

According to some example implementations, the target amplitude is a constant amplitude or an amplitude ratiometrically dependent on a supply voltage of the magnetic sensor apparatus. The term “ratiometric” in electronics means that an unknown variable can be derived from a known ratio of a plurality of other variables to each other. In general, during a ratiometric measurement as a quotient of two variables with the same superposed interference, it turns out that the latter does not influence the measurement. For example, a ratiometric measurement variable (such as a measured amplitude) is independent of a supply voltage (VDD) that may be subject to fluctuations. If, for example, the supply voltage rises unexpectedly at a measurement system, a measurement signal coupled linearly to the supply voltage thus also rises. However, the variable to be measured has not changed. If the measurement signal were then passed to an ADC (analog-to-digital converter) with a fixed reference voltage, the result would be that the ADC provides a code corresponding to a higher measurement variable—incorrect measurement—not a ratiometric system in this case. If the reference voltage of the ADC also rose linearly with respect to the supply, the variable to be measured would not change at the output of the ADC—correct measurement—since in this case there is a ratiometric system. Ratiometric therefore means that, if the measured signal changes due to an interference variable in the system, the comparison variable must change in the same way, such that the change as it were “cancels out” and the change remains with “1/1”. The measurement signal is then multiplied by this fraction. The ratiometry is perfect with 1, is already poorer with 1.15, etc.

According to some example implementations, the magnetic sensor apparatus further includes a second magnetic field sensor circuit which is configured to output a second sensor signal which has a signal amplitude dependent (e.g., a second signal amplitude dependent) on a sensitivity of the second magnetic field sensor circuit. According to some example implementations, the magnetic sensor apparatus may further have a second amplifier circuit which is configured to amplify the second sensor signal and to output a second amplified sensor signal. The control circuit may be configured to use the setting signal to set a supply signal (e.g., a second supply signal) of the second magnetic field sensor circuit and/or a gain (e.g., a second gain) of the second amplifier circuit. Thus, the proposed concept can also be implemented using a replica signal path. The purpose of using a replica signal path may be to ensure that both signal paths always behave identically, even under varying environmental conditions. “Replica circuits” can not only be understood as meaning identical circuits (within the framework of manufacturing tolerances), but also so-called scalable replica circuits. For the latter, for example, a gain factor can differ between the two circuits (e.g., by virtue of different emitter areas or source-drain channels), with an otherwise identical implementation. Replica circuits on an IC have very good synchronization characteristics with each other, which are usually one order of magnitude better than changes over temperature, mechanical stress effects and lifetime drifts. According to some example implementations, the magnetic field sensor circuit and the second magnetic field sensor circuit as well as the amplifier circuit and the second amplifier circuit can thus each be in the form of replica circuits on a common chip.

According to some example implementations, the magnetic field generating circuit may be configured to generate the magnetic field for the magnetic field sensor circuit and the second magnetic field sensor circuit using a common mode signal (common mode WoC). In the case of symmetrical signal transmission, a common mode signal is superimposed on the actual useful signal. Common mode voltage signals can be referred to as common mode voltages. In the case of currents, reference can be made to the common mode current. Common mode signals for symmetrical signal transmission can be DC voltage or DC currents, while useful signals may be complementary AC voltages or currents superimposed on the common mode signal.

According to some example implementations, the magnetic field sensor circuit and the second magnetic field sensor circuit each include at least one Hall sensor. Alternatively, the magnetic field sensor circuit and the second magnetic field sensor circuit may each include at least one xMR sensor. The xMR sensor may be in the form of an xMR sensor bridge circuit and in particular a TMR sensor bridge circuit. TMR sensor bridge circuits may have a particularly pronounced sensitivity drift.

According to some example implementations, the amplifier circuit is in the form of a digital amplifier circuit with an adjustable digital gain. For example, the digital amplifier circuit may have an analog-to-digital converter (ADC) with a reference voltage (e.g., an adjustable reference voltage) that can be adjusted using the control circuit. In addition, a digital gain could also be set after analog-to-digital conversion of the signal.

A further aspect of the present disclosure proposes a method for compensating for sensitivity fluctuations of a magnetic field sensor circuit. The method includes generating a magnetic field, outputting, in response to the magnetic field, a sensor signal using the magnetic field sensor circuit, wherein the sensor signal has a signal amplitude dependent on a sensitivity of the magnetic field sensor circuit, amplifying the sensor signal in order to obtain an amplified sensor signal with an amplified signal amplitude, and setting a supply signal of the magnetic field sensor circuit and/or a gain of the amplifier circuit such that the amplified signal amplitude corresponds to a target amplitude.

Example implementations of the present implementation can solve the problem of the lifetime drift of Hall or xMR sensors (accuracy and stability of the sensitivity), which is caused mainly by mechanical stress. Circuit techniques with a pseudo-random magnetic pilot signal and feedback control loops can be used for stabilization. Example implementations of the present implementation may allow systems with low noise, high bandwidth, high accuracy and low offset at lower cost.

Some examples are now described in more detail with reference to the accompanying figures. However, further possible examples are not limited to the features of these implementations described in detail. These may include modifications of the features, as well as equivalents and alternatives to the features. In addition, the terminology used herein to describe certain examples should not be restrictive for other possible examples.

Throughout the description of the figures, identical or similar reference signs relate to identical or similar elements or features, each of which may be implemented in an identical or modified form, while providing the same or a similar function. In the figures, the thicknesses of lines, layers and/or areas may also be exaggerated for clarity.

If two elements A and B are combined using an “or”, this should be understood to mean that all possible combinations are disclosed, e.g., only A, only B, and A and B, unless explicitly defined otherwise in the individual case. As alternative wording for the same combinations, it is possible to use “at least one of A and B” or “A and/or B”. This applies equivalently to combinations of more than two elements.

If a singular form, such as “a, an” and “the”, is used, and the use of only a single element is neither explicitly nor implicitly defined as mandatory, other examples may also use a plurality of elements to implement the same function. If a function is described below as being implemented using a plurality of elements, further examples can implement the same function using a single element or a single processing entity. It is also understood that the terms “comprises”, “comprising”, “has” and/or “having” in their use describe the presence of the specified features, integers, steps, operations, processes, elements, components and/or a group thereof, but do not exclude the presence or addition of one or more other features, integers, steps, operations, processes, elements, components, and/or a group thereof.

shows an overview illustration of a magnetic sensor apparatusaccording to one example implementation.

The magnetic sensor apparatuscomprises a magnetic field generating circuitwhich is configured to generate a predefined magnetic field. A predefined magnetic field is understood here as meaning a magnetic field for trimming the measurement sensitivity of the magnetic sensor apparatus. The magnetic sensor apparatusalso comprises at least one magnetic field sensor circuitwhich is configured to output a sensor signalin response to the predefined magnetic field, which sensor signal has a signal amplitude dependent on a sensitivity (measurement sensitivity) of the magnetic field sensor circuit. An amplifier circuitof the magnetic sensor apparatusis configured to amplify the sensor signalof the magnetic field sensor circuitand to output an amplified sensor signalwith an amplified signal amplitude on the output side. The amplifier circuitcan also be an analog-to-digital converter with subsequent digital signal processing. A control circuitis configured to use a setting signalto set a supply signal of the magnetic field sensor circuitand/or a gain of the amplifier circuitor the reference of an ADC or the gain of digital signal processing following the ADC such that the amplified signal amplitude corresponds to a target amplitude. The amplified signal amplitude of the amplified sensor signalis thus controlled to the target amplitudeby the control circuit.

The magnetic field sensor circuitmay be in the form of a Hall or xMR sensor circuit, in particular xMR sensor bridge circuit. xMR sensor bridge circuits as a magnetic field sensor circuitare discussed in more detail merely by way of example below.

The circuit components of the magnetic sensor apparatuscan be integrated together on a chip, for example. Numerous different circuit implementations of the magnetic sensor apparatusare possible. Some example implementations are described with reference to the figures below.

shows a magnetic sensor apparatusaccording to a first possible implementation.

The magnetic field generating circuitof the magnetic sensor apparatusshown inhas a current conductorfor conducting a predefined electrical excitation current to flow through the current conductor. The predefined electrical excitation current is provided by a current sourceand has at least a predefined (e.g., known) current intensity. Thus, the magnetic fieldgenerated thereby then also becomes a predefined magnetic field. The current sourcemay have a bandgap reference circuit having a resistor independent of mechanical stress (e.g., metal resistor or silicided polysilicon resistor or a combination of resistors with different stress dependence) in order to generate the electrical excitation current with a constant, stress-independent current intensity. A possible implementation is shown schematically in.

The current conductorcan be, for example, a busbar, a conductor loop or a coil. If a predefined electrical current flows through the current conductor, a predefined magnetic fieldthat is dependent on the electrical current is formed around the current conductor. This predefined magnetic fieldcan then be detected and measured by the xMR sensor bridge circuitwhich is arranged in the vicinity of the current conductor. The measured magnetic field can then be used in turn to infer the current, with the result that the (trimmed) xMR sensor bridge circuitcan be used, for example, as a current sensor. According to some example implementations, the current conductorand the xMR sensor bridge circuitcan be arranged on a common semiconductor chip (WoC).

In the example implementation shown in, the magnetic field generating circuitis configured to generate the magnetic field (e.g., in a trimming mode of the magnetic sensor apparatus) according to an alternating excitation current (AC=alternating current) with a defined amplitude and frequency. Thus, the predefined AC magnetic field can be superimposed, for example, on a DC magnetic field or another AC magnetic field as a useful signal. The alternating excitation current—and thus the AC magnetic field—can be generated according to some example implementations according to a predefined or pseudo-random spreading code which consists of a sequence of pulses or chips (chip=a single elementary modulation state). If the pulse or chip rate (frequency) of the spreading code is sufficiently high, a frequency spectrum of the magnetic field is widened in such a way that it can no longer be distinguished from noise.

The alternating excitation current can be generated or modulated according to some example implementations according to a pseudo-random pilot or spreading signal (pseudo-random signal, PRS). The pseudo-random pilot signal (spreading code) can be a pseudo-random digital n-bit signal (n≥1). The alternating excitation current through the current conductorcan be generated by modulating a DC current. One example of a modulation circuitis shown in. If the switches of the modulation circuitare in the position illustrated in, current flows through the coil (conductor loop). When the switch position is changed, no current flows through the coil. For example, the excitation current can be switched on and off according to a binary pseudo-random pilot signal and can thereby generate a corresponding AC magnetic field.

The predefined magnetic field is measured by the xMR sensor bridge circuitand a sensor signalwhich depends on the magnetic field is output. The sensor signalis shown in this example implementation as a differential (symmetrical) signal. It is understood that the sensor signalcould also be asymmetrical (single-ended). The xMR sensor bridge circuitis in the form of a TMR sensor bridge circuit in this example implementation. It is understood that the xMR sensor bridge circuitcould also be in the form of an AMR or GMR sensor bridge circuit, for example. For TMR sensors, however, the concept of sensitivity drift compensation proposed herein proves to be particularly effective.

The sensor signalof the xMR sensor bridge circuitis supplied to an input of a first amplifier. The first amplifier(e.g., a preamplifier) amplifies the sensor signaland outputs an amplified sensor signalat its output. The amplified sensor signalis shown in this example implementation as a differential (symmetrical) signal. Accordingly, the first amplifieris in the form of a differential amplifier. It is understood that the amplified sensor signaland the first amplifiercould also be asymmetrical (single-ended).

The amplified sensor signalis supplied as a control variable to an analog controllerin the form of a differential amplifier in the example implementation shown. The differential amplifieris in the form of a transconductance amplifier (OTA) here. Other forms of controllers are also conceivable and depend on the specific implementation. For example, the analog control circuit illustrated here could also be digital. That is to say, instead of an analog control loop (with OTAs and capacitors), it is also possible to use a digital control loop (comparator+up/down counter+DAC or PGA (=Programmable Gain Amplifier)) in order to allow low filter frequencies in the loop and thus better suppression of EMC and intermodulation effects.

On the input side of a first (differential) input of the controller (OTA)there is a demodulation circuitwhich is configured to convert the amplified (spread) AC sensor signalinto a (de-spread) DC signal which is then supplied to the first (differential) input of the differential amplifier. In the demodulation circuit, de-spreading is effected by correlating the amplified sensor signalwith the pilot or spreading signal. A target signal corresponding to a target amplitudeis supplied to a second (differential) input of the differential amplifier. This can be a DC signal, such as a DC voltage. A switchcan be used to set whether the target amplitudeis a constant amplitude or an amplitude ratiometrically dependent on a supply voltage VDDext of the magnetic sensor apparatus. In the first case, a voltage divider, which is coupled to the second (differential) input of the differential amplifieron the output side, is coupled to a constant voltage sourceon the input side. The constant voltage sourcecan be a bandgap reference circuit which is configured to generate a constant reference voltage Vconst with the target amplitude. The electrical excitation current for the magnetic field generating circuitwith a constant current intensity can also be derived from the constant voltage sourceby virtue of the latter having, for example, a resistor independent of mechanical stress in order to generate an excitation current independent of mechanical stress. In the second case (ratiometric), the voltage divideris coupled to the supply voltage VDDext of the magnetic sensor apparatuson the input side via the switch.

The differential amplifier (OTA)is configured to output the setting signalfor the first amplifier, based on a difference between its two input signals (amplified signal amplitude and target amplitude). A differential voltage at the two inputs of the differential amplifieris thus converted into a proportional output current (setting signal) which is used to set the gain of the first amplifiersuch that the differential voltage at the two inputs of the differential amplifierbecomes zero if possible—the amplified signal amplitude and the target amplitude are therefore substantially the same.

Between the first amplifierand a second, downstream amplifier(e.g., output amplifier), the magnetic sensor apparatushas a removal circuitwhich is configured to remove the pilot or spreading signal at least partially from the amplified sensor signal. For this purpose, the removal circuitin the example implementation shown has a 1-bit DACfor the pilot or spreading signal and is configured to subtract an output signal of the 1-bit DACfrom the amplified sensor signal. In particular, the 1-bit DACconverts the digital 1-bit pilot or spreading signal into a (differential) analog signal. This analog spreading signal is then subtracted from the amplified sensor signalwhich also contains the spreading signal. If the amplitude of the analog spreading signal at the output of the DACand the amplitude of the spreading signal contained in the amplified sensor signalmatch, substantially complete removal of the spreading signal from the amplified sensor signaltakes place. The signal remaining after this corresponds, for example, to a measured DC magnetic field and can be amplified by the output amplifierfor further processing. It is understood that the modulation circuitand the removal circuitshould be synchronized.

A possibly remaining small spreading signal at the input of the output amplifier, which comes from a mismatch of the analog spreading signal at the output of the DACto the spreading signal contained in the amplified sensor signal, can be compensated for by a second demodulator (not shown) which finally sets an amplitude of the DAC. Additionally, or alternatively, possible remaining harmonics of the spreading signal at the output can be compensated for by a low-pass filter at the output of the DACor at the input of the output amplifier. This low-pass filter can have the same bandwidth as the preamplifier.

In summary,shows an implementation of TMR sensor stabilization of the sensitivity with a pseudo-random (spread spectrum) pilot tone for wire-on-chip (WoC) and pilot tone compensation at the output amplifier. The control loop may have a controllable amplifier or digitally programmable amplifier.

shows a further implementation of a magnetic sensor apparatuswhich differs from the magnetic sensor apparatusonly by the implementation of the control circuit. Whereas the gain of the amplifiercan be controlled using the setting signalin the control circuit in, a supply voltage of the xMR sensor bridge circuitcan be controlled using the setting signalin the control circuit according to, as a result of which the sensitivity of the TMR bridge sensor can be proportionally controlled. The gain of the (pre-) amplifieris constant here.

The principle of the circuits according tocan be summarized as follows. A pseudo-random pilot tone is inserted into the WoC of an xMR bridge. The amplitude of the obtained WoC signal is demodulated downstream of the preamplifierand is compared with a target amplitude. A control signalis derived from this comparison, which control signal controls either the supply voltage of the xMR bridgeor the gain of the preamplifier. The pilot signal is then subtracted from the signal path again using a 1-bit DAC in order to obtain a clean output signal without superimposed pilot tones.

shows an implementation of a magnetic sensor apparatuswith two signal paths. A first signal path comprises a first xMR sensor bridge circuit-which outputs a first (differential) sensor signal-in response to a magnetic field to be measured. The first (differential) sensor signal-is supplied to a (differential) input of a first controllable amplifier-in order to obtain a first amplified sensor signal-at its output. A second signal path comprises a second xMR sensor bridge circuit-which outputs a second (differential) sensor signal-in response to a predefined trimming magnetic field. The trimming magnetic field can in turn be an AC magnetic field according to a spreading code. The second (differential) sensor signal-is supplied to a (differential) input of a second controllable amplifier-in order to obtain a second amplified sensor signal-at its output. The second signal path is a replica signal path to the first signal path. The first and second xMR sensor bridge circuits-,-are therefore replica circuits (e.g., formed as a first replica circuit formed on a common chip). The first and second amplifiers-,-are also replica circuits (e.g., formed as a second replica circuit formed on the common chip). The replica circuits can be implemented together on a chip (die).